When manufacturing infrared focal plane array detectors, one of the critical components is the zinc sulfide film which is deposited on the semiconductor substrate. The zinc sulfide (ZnS) film serves as the insulator in the metal-insulator-semiconductor (MIS) structures forming the detectors. Additionally, the MOCVD ZnS films are used as interlevel insulators and anti-reflection coatings in the manufacture of infrared device structures. Control of the thickness of the ZnS films is critical to the performance of the IR device because the capacitance of a detector element is inversely proportional to the zinc sulfide film thickness. Accordingly, the zinc sulfide deposition process needs to provide exceptional film thickness control and reliability. Additionally, with the advent of slice-level processing, film thickness uniformity across the entire slice is necessary for high volume production.
In the past, zinc sulfide films have been deposited by physical vapor deposition (PVD) processes. Even though the electrical and optical properties of zinc sulfide are well suited for use in an infrared detector, previously developed PVD processes of film deposition have produced films with an inherently high density of defects and with poor step coverage, and are thus not well suited for integrated circuit (IC) fabrication.
When using PVD processes, an evaporator or a sputtering system are generally utilized which creates undesirable side effects, such as particle accumulation, on the evaporator chamber walls. This causes excessive flaking of the particles from the reactor walls and leads to the incorporation of particles into the growing ZnS film which contaminates the semiconductor substrate being processed Ultimately, such particle contamination leads to device degradation or failure resulting from electrical shorts or opens.
More recently, the deposition of zinc sulfide films has been accomplished using a metal organic chemical vapor deposition (MOCVD) process by combining hydrogen sulfide and dimethylzinc gases. Despite the advancement in the MOCVD process, an apparatus or process had not been developed which could accomplish the deposition of uniform films at the low deposition temperature necessary for use in the production of infrared detectors. Prior art exists for MOCVD ZnS film deposition using hydrogen sulfide and dimethylzinc at substrate temperatures between 250.degree. C. and 500.degree. C. to produce very high quality epitaxial ZnS layers. The reactant gases in this case are diluted in H.sub.2 or He carrier gases. The substrate temperatures required for the epitaxial-quality ZnS films would cause serious degradation of the HgCdTe substrate commonly used as the infrared detector material in focal plane devices, primarily due to Hg loss from the HgCdTe. A need has therefore arisen for an apparatus and process which can easily deposit zinc sulfide films using dimethylzinc gas and hydrogen sulfide gas at a low temperature (less than 100.degree. C.) with minimal contamination, and which is capable of being utilized in the high volume production of infrared focal plane detectors. While the discussion focuses on M-I-S detector structures, the uses of MOCVD low temperature ZnS films are not limited to such structures. This technology is applicable to other IR detector types including, but not limited to, photodiodes and photoconductors.